Specific Gravity And Viscosity Of Liquids

Specific gravity is the heaviness of a substance compared to that of water, and it is expressed without units. In the metric system specific gravity is the same as in the English system. If something is 7.85 times as heavy as an equal volume of water (such as iron is) its specific gravity is 7.85. Its density is 7.85 grams per cubic centimeter, or 7.85 kilograms per liter, or 7.85 metric tons per cubic meter. In relationship to liquids, the term specific gravity is used to describe the weight or density of a liquid compared to an equal volume of fresh water at 4°C (39° F). If the liquid you are comparing will float on this water it has a specific gravity of less than one (1). If it sinks into the fresh water the specific gravity is more than one. As you have already guessed fresh water at 4°C (39° F) has been assigned a value of one (1).

It is important that you do not confuse specific gravity with viscosity which is a measurement of a fluids resistance to pouring, though there is often what is perceived to be an obvious relationship. Thick liquids are said to have a high viscosity and thin liquids a low viscosity. Like specific gravity, viscosity can be altered by a change in temperature, but unlike specific gravity it can also be altered by agitation. The really important thing to keep in mind is that there is no correlation between viscosity and specific gravity. We measure specific gravity with a hydrometer. It consist of a glass cylinder with a rubber bulb on top, and a float positioned inside the glass tube. The float is calibrated to float on fresh water so if the fluid you are testing has a higher specific gravity, the float will raise in the liquid and at a lower specific gravity it will sink lower in the liquid. This is the same instrument that we use to tell if your automobile battery is fully charged. Another version will determine the concentration of anti-freeze in an automobile radiator. You can observe the little balls floating in the tube.

In our table below, we have given various liquids' characteristics with the temperature modified to one of more of a real world value. As a rule of thumb, multiply the specific gravity of a liquid by 8.34 to find the weight of a gallon of that liquid.